[strong] Implement strong mode semantics for the count operation.

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
diff --git a/src/x64/full-codegen-x64.cc b/src/x64/full-codegen-x64.cc
index 0b19edd683084bd92d7f67ebdff57ec6f3d43d12..0c23096c5bfef0f986dc4b874c2e61f12a9596aa 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -4977,10 +4977,11 @@ void FullCodeGenerator::VisitCountOperation(CountOperation* expr) {
     __ jmp(&stub_call, Label::kNear);
     __ bind(&slow);
   }
-
-  ToNumberStub convert_stub(isolate());
-  __ CallStub(&convert_stub);
-  PrepareForBailoutForId(expr->ToNumberId(), TOS_REG);
+  if (!is_strong(language_mode())) {
+    ToNumberStub convert_stub(isolate());
+    __ CallStub(&convert_stub);
+    PrepareForBailoutForId(expr->ToNumberId(), TOS_REG);
+  }
 
   // Save result for postfix expressions.
   if (expr->is_postfix()) {
@@ -5021,6 +5022,9 @@ void FullCodeGenerator::VisitCountOperation(CountOperation* expr) {
   patch_site.EmitPatchInfo();
   __ bind(&done);
 
+  if (is_strong(language_mode())) {
+    PrepareForBailoutForId(expr->ToNumberId(), TOS_REG);
+  }
   // Store the value returned in rax.
   switch (assign_type) {
     case VARIABLE: